This invention relates to photometric devices and, more particularly, to photometric devices suitable for use in single lens reflex cameras.
Photometric devices are utilized by single lens reflex cameras to either provide an indication of how the camera should be adjusted or automatically control certain camera adjustments, such as shutter speed, for example, for a particular lighting condition. In recent years attempts have been made to extend the operative range of photometric devices to increasingly lower brightness levels. Spot photometry (center area priority photometry) has become an increasingly popular technique designed to achieve this result. However, at the present time light receiving elements suitable for operating properly at lower brightness levels are difficult to produce. Further, prior art attempts to measure the light from a subject by arranging a photometric device behind a pentaprism have not been as satisfactory as desired because an insufficient photoelectric output has been obtained.
Therefore, it is an object of this invention to provide a new and improved photometric device.
It is another object of this invention to provide a photometric device that develops a large photometric output even at low brightness levels, and allows spot photometry.
In the past, increased photoelectric output has been obtained by placing a lens system in front of a light receiving element. However, such a system makes it difficult to freely select the brightness range of the light to be measured. A prior art method of eliminating this disadvantage is to mount a light (optical) stop in front of the light receiving element and, thereby, control the photometric range. This method, however, is inefficient and does not effectively improve the accuracy of the photometric operation. More specifically, this system is ineffective because the light receiving element is not utilized over its entire light receiving surface; rather, it is used only over the limited area defined by the light stop. Because the dark current flowing through the light receiving element is generally proportional to the tip area of this element and varies largely with temperature, as the brightness of the subject becomes lower, the dark current adversely affects the photometric operation as a noise signal. This noise signal reduces the so-called signal-to-noise (S/N) ratio and, hence, the accuracy of the photometric operation. Consequently, it is desired to eliminate the light stop, design the light receiving element as small as possible and use all of its light receiving surface.
Therefore, it is a further object of this invention to provide a photometric device that utilizes a small light receiving element, yet develops a sufficiently large photoelectric output even in low brightness regions to provide a photometric device suitable for use in a single lens reflex camera.
It still another object of this invention to provide a photometric device suitable for use in a single lens reflex camera that does not require the inclusion of a light stop.